1. Field of the Invention
The present invention relates to an image processing apparatus, and more particularly relates to an image processing apparatus for stereographically photographing the ocular fundus of a subject's eye with a prescribed parallax via an ocular fundus photographic optical system and for processing the resulting parallax images for three-dimensional shape measurement and three-dimensional display.
2. Description of the Prior Art
Conventionally known are a fundus camera and other image processing apparatuses for photographing a stereo image of the ocular fundus of a subject's eye in order to understand the shape of the ocular fundus of the subject's eye for the purpose of diagnosing glaucoma or performing other examinations. For example, an apparatus is known in which a photographic stop inside an optical system in a fundus camera is moved to different positions that are offset to the left and right (or up and down) from the optical axis to photograph the ocular fundus of the subject's eye at the offset stop positions, thereby providing stereographically viewed images (Japanese Laid-open Patent Application No. 1998-75932)
Image processing in which the ocular fundus is three-dimensionally measured can be carried out in the following manner. For example, the left and right (or upper and lower) images thus stereographically photographed are subjected to pattern matching in order to search corresponding points in both the images. If the corresponding points can be found, the basic principles of triangulation is used to calculate the coordinates along the z axis (the axis parallel to the optical axis) and the x and y axes (the two orthogonal axes that are parallel to the image surface) of an object point that is conjugate with the corresponding points. The three-dimensional coordinate values of a sufficient number of object points are calculated to produce a three-dimensional model of the ocular fundus of a subject's eye.
The left and right parallax images thus photographed are three-dimensionally displayed (3D display) via a 3D monitor or the like using various methods. In other words, the photographed left and right parallax images are independently presented to the left and right eyes of an observer (examiner). This allows the examiner to stereoscopically observe the state of the photographed ocular fundus of a subject's eye.
Image distortions are present in the optical system of a photography apparatus such as a fundus camera. For example, an image such as reference numeral 151 of FIG. 5 is photographed when a grid pattern such as graph paper placed on a flat surface is photographed. These distortions include those that are presumed to exist in advance in terms of the design of the optical system, as well as those that are produced by errors in manufacturing that vary depending on individual optical systems.
If stereographic photography and a three-dimensional image process are carried out in the manner described above using an optical system that has such image distortions, the z-axis coordinates obtained on the line P-P′, for example, may possibly become deformed as shown by reference numeral 153 despite the fact that the z-axis coordinates should be linearly measured in the manner indicated by reference numeral 152 in the case of a grid pattern placed on a flat plane.
Generally, image distortions such as described above must be corrected in three-dimensional measurement in which a stereographically photographed image is used. Without correction a three-dimensional shape measured would be different than the actual photographic target. In view of this possibility, a configuration is used in which the image data is subjected to affine transformation using parameters that correspond to distortions of the optical system estimated in advance. In the case of ocular fundus photography, a configuration is known in which a stereographically photographed image is corrected under the presumption that the ocular fundus is an ideal sphere (Japanese Laid-open Patent Application No. 2002-34924).
However, it is difficult to say that such a conventional correction process accurately reflects the characteristics of the optical system used in actual photography, and it is highly possible that some type of error will occur in the three-dimensional measurement results of the ocular fundus. In clinical applications such as retinal shape analysis of the ocular fundus and papillary shape analysis of the optic nerve, there is risk of misdiagnosis depending on the extent of errors, and there is a need for an image process that can accurately correct for distortions in the optical system from the stereo photographic image.
For example, in the case of the grid pattern placed on a plane in FIG. 5, there is a need to be able to accurately correct in the manner of reference numeral 152 the three-dimensional shape on the line P-P′ that has been distorted and measured as reference numeral 153.
It is believed that the shape data of the three-dimensional model of the ocular fundus of a subject's eye should be corrected in order to remove the effects of distortions in the optical system in this manner, and that the ocular fundus image thus photographed should be corrected. When left and right parallax images thus photographed are three-dimensionally displayed and the photographed ocular fundus of the subject's eye is stereographically presented to the examiner, it is preferred that the image also be corrected for 3D display so that the examiner can observe (perceive) the shape of the ocular fundus of the subject's eye by using the shape that reflects the correction of the shape data. When the photographed ocular fundus image is used for recording, i.e. printed on paper or the like, or recorded as data on an HDD or the like, it is naturally desirable that the image be corrected in the same manner.
It is therefore an object of the invention to provide an image processing apparatus for stereographically photographing the ocular fundus of a subject's eye and subjecting the resulting photographic image data to a three dimensional measuring process, wherein consideration is given to distortions in the optical system that is used in actual photography, an accurate three dimensional measuring process involving the fundus image can be performed, an accurate fundus image that corresponds to the three-dimensional measurement results can alternatively be provided, and the examiner can accurately evaluate the stereo shape of the ocular fundus of the subject's eye.